Had this problem on my XP PC. Trying to be clean and green so I put the PC into hibernation when I'm away for any length of time. (To do this, just press the power switch and then choose the Hibernation option.)
It stayed in hibernation for about 10 minutes and then came out of it on it's own accord.
Mr. Google to the rescue and some research showed the answer here.
The trick is to select the "Only allow management stations to bring the computer out of standby check box" option as described in the article. Problem solved.
Note: The title of the article i.e. "The computer may unexpectedly resume from standby or hibernation and then automatically return to standby or hibernation after two minutes" is somewhat misleading because my PC didn't return to hibernation but the fix still worked.